Antiphospholipid antibody (aPL) is closely associated with recurrent fetal death, thrombosis, and thrombocytopenia. The mechanisms by which the antibody causes the complications remain unexplained. The specific goals of this proposal are: 1. To characterize the idiotypes of human polyclonal and mouse monoclonal aPL antibodies and to define "pathogenic" public idiotypes. Anti-id will also be used to detect aPL in the placenta and other tissues by immunofluorescence. 2. To define the pathophysiology aPL. The effect of aPL on the activity of the placental anticoagulant protein (PAP) will be analyzed by ELISA as well as PL-dependent coagulation assays, KCT and APTT. The interaction of aPL with the lupus cofactor (apolipoprotein H) will be evaluated by ELISA and KCT and APTT coagulation assays. The levels of cofactor in patients' sera will be determined by ELISA and the co factor isoforms identified by isoelectric focusing. Associations between PAP levels, cofactor levels, cofactor isoforms and clinical complications will be assessed. 3. To explore the pathogenic nature of the antibody in spontaneous (MRL/lpr) and induced mouse models. Human polyclonal and mouse monoclonal antibodies will be administered to normal pregnant mice, or antiphospholipid antibodies will be induced in normal mice by immunization with apolipoprotein H. The effect on pregnancy (number of pups) and hematology (platelets, thrombosis) will be determined. To suppress aPL production, anti-id antibodies will be administered to MRL/lpr mice and circulating aPL levels, platelets and pregnancy outcome monitored. 4. To determine the VH sequences of aPL genes so as to define the genetic origins of aPL in MRL mice. The characterization and functional evaluation of PAP, lupus cofactor, and anti-id antibodies will determine whether aPL antibodies are directly involved in the pathogenesis of the syndrome and may uncover their pathogenesis. Identification of pathogenic idiotypes and the genetic origin of aPL may provide tools to identify patients at risk.